Regiospecific analysis of neutral ether lipids by liquid chromatography/electrospray ionization/single quadrupole mass spectrometry: validation with synthetic compounds

A reversed-phase high-performance liquid chromatography (HPLC) method with on-line electrospray ionization/collision-induced dissociation/mass spectro metry (ESI/CID/MS) is presented for the regiospecific analysis of synthetic reference compounds of neutral ether lipids. The reference compounds were characterized by chromatographic retention times, full mass spectra, and fr agmentation patterns as an aid to clarify the regiospecificity of ether lip ids from natural sources. The results clearly show that single quadrupole m ass spectroscopic analysis may elucidate the regiospecific structure of neu tral ether lipids. Ether lipid reference compounds were characterized by fi ve to six major ions in the positive ion mode. The 1-O-alkyl-sn-glycerols w ere analyzed as the diacetoyl derivative, and showed the [M - acetoyl](+) i on as an important diagnostic ion. The diagnostic ions of directly analyzed 1-O-alkyl-2-acyl-sn-glycerols and 1-O-alkyl-3-acyl-sn-glycerols were the [ M - alkyl](+), [M + H - H2O](+) and [M + H](+) ions. Regiospecific characte rization of the fatty acid position was evident from the relative ion inten sities, as the sn-2 species had relatively high [M + H](+) ion intensities compared with [M + H - H2O](+), whereas the reverse situation characterized the sn-3 species. Furthermore, corresponding sn-2 and sn-3 species were se parated by the chromatographic system. However, loss of water was promoted as fatty acid unsaturation was raised, which may complicate interpretation of the mass spectra. The diagnostic ions of directly analyzed 1-O-alkyl-2,3 -diacyl-sn-glycerols were the [M - alkyl](+), [M - sn-2-acyl](+) and [M - s n-3-acyl](+) ions. Regiospecific characterization of the fatty acid identit y and position was evident from the relative ion intensities, as fragmentat ion of the sn-2 fatty acids was preferred to the sn-3 fatty acids; however, loss of fatty acids was also promoted by higher degrees of unsaturation. T herefore, both structural and positional effects of the fatty acids affect the spectra of the neutral ether lipids. Fragmentation patterns and optimal capillary exit voltages are suggested for each neutral ether lipid class. The present study demonstrates that reversed-phase HPLC and positive ion ES I/CID/MS provide direct and unambiguous information about the configuration and identity of molecular species in neutral 1-O-alkyl-sn-glycerol classes . Copyright (C) 2001 John Wiley & Sons, Ltd.